In a storage apparatus, a solid-state drive (SSD) or a hard disk drive (HDD) is mounted as a storage device. The HDD is a storage device which reads information stored in a disk coated with a magnetic material, by rotating the disk at a high speed and moving a magnetic head. The SSD is a storage device that uses a NAND-type flash memory as a storage medium.
A storage device using a NAND-type flash memory, such as an SSD, includes a plurality of memory blocks. The plurality of blocks are divided into a main area used for Write to the blocks and Read from the blocks, and a spare area used as a reserve area of the main area. In the SSD, a failure of a block unit occurs due to aging or the like. A failure caused by deterioration is previously assumed. Therefore, a medium such as an SSD has a spare area including spare blocks. By executing firmware, a processor of the storage apparatus regards a block in which a failure has occurred as a failure block, and replaces the failure block with a spare block within a spare area. The SSD maintains an initial capacity of a medium by replacing the capacity of the failure block with the spare block.
In terms of performance and life, a spare area of an SSD is used for background processing such as wear leveling, garbage collection, or the like. When there is a block with a high usage frequency within the SSD, the block with the high usage frequency (writing frequency) quickly reaches the end of life. A wear leveling technology is a technology in which a block with a low usage frequency is used to replace a block with a high usage frequency so as to achieve a levelling of usage frequencies of respective blocks.
Data in an SSD is written in the SSD by a unit called a page. However, deletion of data is performed by a unit of a block composed of a plurality of pages. A garbage collection technology is a technology in which when data on a page within a block is no longer needed, other data within the corresponding block, which is still needed, is transferred (written) to another block.
A method of extending a life of each of storage tiers within an automated storage tiering (AST) environment has been known. According to this method, a storage control device allocates a storage tier corresponding to a policy specified for an attribute or a volume of data in order to prolong a life of each of the storage tiers.
There is known a storage system in which the life of a flash memory is considered. The storage system stores and manages management information of an online flash memory. In a case where a portion or whole online flash memory is placed in an offline state, existing data stored in an area serving as a management area at the time of offline in the flash memory is moved to an area other than the management area, and the management area is created accordingly. Management information of the flash memory is then written in the created management area, and the flash memory is placed in an offline state.
Related techniques are disclosed in, for example, Japanese Laid-Open Patent Publication No. 2007-115232 and Japanese Laid-Open Patent Publication No. 2008-192061.
Background processing such as wear leveling, garbage collection, or the like is carried out during a process of writing to the SSD. On the other hand, the background processing is not carried out during a process of reading from the SSD. Therefore, an SSD to which a Read access is frequently made is placed in a state where the SSD keeps an unnecessary spare area that is not used, since the background processing is not carried out.